10 Answers
10

As previously discussed RPi does not support PoE. And yes you could use a PoE module to hookup power to GPIO. But if you're not up for hardware hacking you could just get a Ethernet/USB power splitter off the shelf.

For up-to-date product list Google is your friend. Searches to use include:

I know that we don't usually post links to products, but they are kinda hard to find. And may be worth around keeping until RPi accessories become common place. It's worth noting that the first one even lists RPi compatibility!! :)
– Maria ZverinaJun 26 '12 at 13:44

As the post says, most PoE adapters will require a 2.1mm-to-USB cable or adapter. These can be expensive and/or hard to obtain. As an alternative it's also possible to use a combination of cables (e.g. 2.1mm-to-USB-A plus USB-A to micro-USB) that may be cheaper/easier to come by - more details here.
– psmearsAug 12 '12 at 17:20

Note that recently there have been reports of some issues with the PoE HAT on some 3B+ units. Depending on the source of a critical component on the 3B+, the PoE HAT may only be able to supply 200mA (1W) of power to the USB ports. Thus a keyboard or mouse might be fine, but anything more power hungry would fail. If this problem affects you, I would suggest that you ask for your Raspberry Pi 3B+ to be replaced under warranty with an unaffected 3B+.

EEVblog #1122 investigated this, and provides some great close up views. It's disturbing how hot it gets though.

Unlike after market PoE HATs for models before the 3B+, the new PoE HAT uses the on-board Ethernet port, so you won't need you to plug your Ethernet cable into the HAT and then link the HAT back to the Raspberry Pi Ethernet port with another cable.

The PoE HAT does require a new 4-pin connector on the 3B+ board however, circled here:

I have tested the POE connector with a multimeter, as I expected each pin on the POE header corresponds to a pair on the Ethernet connector. I include a picture below of the pin mapping I have determined.

The 3b+ has a poe magjack and an extra connector which goes to the poe hat. I'm not 100% sure but I think the conector just brings out the raw center taps and then the hat does everything else.
– Peter GreenMar 16 '18 at 2:52

That's what I was assuming @PeterGreen unfortunately the schematic they provide for the 3B+ is not a complete schematic, so I can't see how the Ethernet port is wired differently.
– Mark BoothMar 16 '18 at 7:51

Should be easy enough to confirm with a mulitmeter once you have your hands on a Pi3 b+.
– Peter GreenMar 16 '18 at 14:33

@PeterGreen That's the only sane thing that can be done :)
– Kuba OberMar 16 '18 at 18:44

I will answer with the assumption that you want to have a solution that interfaces directly with the Raspberry Pi's ethernet connector.

First of all, the RPI uses a magjack, not merely an RJ-45 connector. The best I can tell from the B board, they use an EDAC A63-113-300P131 magjack.

The manufacturer's schematic is below, followed by the connector symbol from RPI-B schematic, and the pinout. They agree, although the manufacturer and RPI folks number the pins differently.

A compliant PoE solution needs access both to the primary winding center taps, and to RJ45 pins 4, 5, 7 and 8. Those are (of course!) not available outside the magjack in use.

One hacky solution is to desolder the magjack, remove the 75 Ohm resistors, and bring out the center taps and the pins 4,5,7 and 8. There's at least one person who did just that, and wired it to the PEM1205 PoE module.

A less hacky solution would be to find a PoE magjack with the same mechanical outline and pinout, and solder it in place of the original magjack. PoE magjacks can have built-in rectifiers, requiring only 2 pins to pass the power to the PoE supply - thus requiring only 8 pins total (6 for data, 2 for power). Such 8 pin magjacks do in fact exist.

Unfortunately, all the magjacks that DigiKey currently stocks - those by Bel Fuse, TRP, Wurth and Pulse, have wrong pinouts. Some would mechanically fit, perhaps with a pin or two cut, but the pinout is all wrong.

The only solutions I can think of, that would still retain the small form-factor of RPI, and its 3D bounding box, would be to unsolder the original magjack and:

Add a small interposer board between a different magjack and the RPI. The board would sit directly between the magjack and the RPI, moving the magjack about 0.07 inches higher. It would expose the 5 pins needed for PoE operation (shield, center taps, 4+5, 7+8), or the 3 pins if a rectifier magjack is used. Cons:

The RPI's case needs to be modified to accommodate the elevated magjack.

The connector's pins have to be trimmed before being soldered into the interposer board, since the board must sit flush with the RPI board.

If the PoE board is to be soldered directly into the interposer, the interposer will hang outside of the RPI's outline.

Add an adapter board with tall pins that go behind the new magjack, with the magjack soldered upside down and moved out. Pros:

The cases don't need to be modded - the magjack will fit through existing case hole.

The adapter board can accommodate the PoE board itself.

Cons:

The magjack will stick out by ~0.2 inches. The extra space behind it is needed for the pins that go between the RPI's magjack signal pads and the adapter board.

The 0.5 inch long interposer pins are inserted into the ethernet data pairs. This presents an impedance mismatch and may not make the solution 802.3 spec compliant.

The PoE boards are available in multiple output voltages. You can use a 5V board and feed its output directly into P1 connector's pins 2 and 4. If you push more than 5V into those pins, you'll destroy the PI, so be careful if you wish to implement one of the suggestions.

If you want a Raspberry Pi PoE solution that conforms to IEEE 802.3af Standard (PoE), then try the Xtronix Raspberry Pi interface. It is not cheap, but it allows the Pi to be powered via the ethernet cable from a standard PoE hub/switch. The IEEE 802.3af PoE standard defines how devices can be powered over the CAT 5 Ethernet cable.One of the issues the standard addresses is that of safety. A hub/switch that meets the standard allows Poe and non-Poe devices to be mixed safely in the network.

If you want to power your Pi via PoE, in addition to the PoE Hat answer by Mark Booth, you could use a PoE Splitter which means NO modifications are required to the Pi itself. Indeed, using a splitter means leaving the Pi free for a different hat unrelated to powering it. Also, the PoE Splitter is independent of the Pi and should work with any model of Pi. It just diverts power from the switch to a MicroUSB lead on the device and supplies it to the Pi's female MicroUSB port.

The Raspberry Pi 3B+ does not work with PoE wirings where two pairs are used for 10/100BaseT and two pairs are used for power. It needs all 8 wires connected.

However, if you have a proper 802.3af injector you can retrieve the injected power via the pins on the PoE connector.

I used a MikroTik GB PoE Adapter with a 24V supply, standard polarity (jacket-,core+). In my case minus came out to the pin next to the 'pen' marking and plus on the pin next to the 'run' marking (left front and right front respectively):

I connected these to a stepdown DC-DC converter (search for HW-636 on ebay).

The essence seems to be that you only need 4 of the 8 wires in an RJ45 cable for internet. You can then use the others for power. With this solution it seems you need to manually branch out the wires for power into a voltage regulator chip at the Pi end and use a small POE splitter device at the power/network end.